where the mass \(\mu \)  is a constant. For concreteness, we consider the conformal coupling \(\xi =1/6\) , which is most natural [1]}, [2]}, but our conclusions apply for any value of \(\xi \) , including minimal coupling \(\xi =0\) . If we define the \(\eta \) -independent conformal metric \(\tilde{g}_{\mu \nu }=a^{-2}\,g_{\mu \nu }\)  and corresponding field \(\tilde{\varphi }=a\varphi \) , Eq. (REF ) becomes
\(-\tilde{\Box }\tilde{\varphi }+\frac{1}{6}\tilde{R}\tilde{\varphi }+\tilde{\mu }^{\;\!2}\tilde{\varphi }=0\) 
